Possible presence of the difference peptide in alkali light chain 1 of fish fast skeletal myosin

1. Presence of N-terminal peptide ("difference peptide") in alkali light chain 1 (A1) of fish fast skeletal myosin was examined by comparing two kinds of light chain-based myosin subfragment 1 (S1) isozymes from the yellowtail Seriola quinqueradiata. 2. On tryptic digestion, A1 was cleaved to a smaller fragment (mol. wt decrement by 2000) along with the cleavage of S1 heavy chain, while A2 was resistant to trypsin. Two-dimensional gel electrophoresis showed that A1 released a basic peptide by tryptic digestion. 3. Both S1 isozymes showed clear kinetic differences in actin-activated Mg-ATPase activity, suggesting a higher affinity of A1 for actin. Affinity of A2 for heavy chain was also estimated to be about 2-fold higher than that of A1, as judged by the model experiments in which rabbit S1 isozymes were hybridized with heterologous alkali light chains.